US20070115174A1 - Direction finding and mapping in multipath environments - Google Patents
Direction finding and mapping in multipath environments Download PDFInfo
- Publication number
- US20070115174A1 US20070115174A1 US11/480,084 US48008406A US2007115174A1 US 20070115174 A1 US20070115174 A1 US 20070115174A1 US 48008406 A US48008406 A US 48008406A US 2007115174 A1 US2007115174 A1 US 2007115174A1
- Authority
- US
- United States
- Prior art keywords
- transmitter
- signal
- receiver
- multipath
- path
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Images
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S3/00—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
- G01S3/02—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using radio waves
- G01S3/14—Systems for determining direction or deviation from predetermined direction
- G01S3/46—Systems for determining direction or deviation from predetermined direction using antennas spaced apart and measuring phase or time difference between signals therefrom, i.e. path-difference systems
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S3/00—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received
- G01S3/02—Direction-finders for determining the direction from which infrasonic, sonic, ultrasonic, or electromagnetic waves, or particle emission, not having a directional significance, are being received using radio waves
- G01S3/04—Details
- G01S3/10—Means for reducing or compensating for quadrantal, site, or like errors
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/0205—Details
- G01S5/0218—Multipath in signal reception
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S5/00—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations
- G01S5/02—Position-fixing by co-ordinating two or more direction or position line determinations; Position-fixing by co-ordinating two or more distance determinations using radio waves
- G01S5/04—Position of source determined by a plurality of spaced direction-finders
Definitions
- the present invention relates to the location of stationary and mobile transmitters by direction finding from mobile vehicles and, in particular, in a multipath environment.
- an emitter of electromagnetic radiation such as a radar system, a communications facility or device or an emergency beacon or transmitter.
- Typical applications may include, for example, military signal intelligence (SIGINT) and electronic intelligence (ELINT) operations for locating radar or communications facilities, and air, land and sea rescue operations wherein it is necessary to locate an emergency beacon or transmitter, such as used in aircraft and vessels, or communications devices ranging from conventional or emergency radio devices to cell phones.
- SIGINT military signal intelligence
- ELINT electronic intelligence
- Such applications and operations are characterized by common requirements that are, in turn, imposed by general, common characteristics of the target emitters to be located and the situations or circumstances under which the target emitters are to be located.
- the signal transmitted by a target emitter may be of relatively low power, as in the case of emergency beacons or emergency radios, or may be masked, distorted or effectively reduced by terrain or weather conditions, and such conditions may be intentionally imposed in, for example, military or otherwise hostile situations.
- the time available or permissible for locating a target emitter may be limited in both military and civil situations, that is, and for example, in military counter-measures operations or in search and rescue operations, and the resources available for target emitter location may be limited.
- a system for locating target emitters it is generally necessary or desirable for a system for locating target emitters to be mobile, that is, to be readily transportable into the general geographical location of a target emitter on an aircraft, vehicle or vessel, both to bring the locator system into range of the target emitter and to allow the locator system to search as large an area as possible in the minimum time. It is also desirable that a locator system be transported and employed in and from a single platform, whether an aircraft, vessel or vehicle, as the use of a single platform reduces the system cost, reduces demand on frequently limited resources and allows a greater area or number of areas to be searched when multiple platforms are available. A single platform system also eliminates the complexity and time delays inherent in deploying and coordinating multiple cooperatively operating platforms.
- the locator system must be capable of identifying the geographic location of a target emitter with the greatest possible accuracy as insufficient accuracy in locating a target emitter may render counter-measures ineffective in military situations and may unacceptably delay locating or reaching the target emitter in civil situations, such as search and rescue operations, particularly in difficult terrain or weather conditions.
- the locator system should be capable of locating as wide a range of target emitter types as possible, and correspondingly over as wide a range of the electromagnetic spectrum as possible, to allow a given locator system to be employed in as wide a range of applications and situations as possible.
- DF direction finding
- Each “cut” is an attempt to determine the direction of the emitter relative to the locator platform at the point the “cut” is taken by using an amplitude or phase detecting directional antenna and receiver array to determine the direction of the strongest signal component or the phase gradient, that is, the direction of propagation, of the wavefront of the received signal.
- Successive DF cuts may be used to determine a Line of Bearing (LOB) “fan” of DF cuts, with the location of the target emitter being taken as the point of intersection of the DF bearings forming the LOB fan.
- LOB Line of Bearing
- multipath sources typically appear to surround the receiving unit and to have effectively random radiation patterns and arise from the reflection or refraction of the transmitted signal by “scatterers”, which may be any element of the environment capable of reflecting the original signal or of refracting the original signal around themselves.
- Conventional direction finding systems typically employ two or more receiving antennas spaced apart from one another along a “baseline” and compare the amplitudes or phases of the signals received at the antennas to determine the direction to the transmitter.
- This method is, however, historically subject to systemic errors for a number of reasons. For example, if the antennas are spaced too closely there will be correlation between the multipath components of the received signal, and between the multipath components and the direct arrival component, resulting in an induced multipath bias error that cannot be “washed out” even by time integration of the received signal components. If, however, the antennas are spaced too far apart, such as more than one wavelength apart, the multipath and direct arrival components will be decorrelated, but there will be phase ambiguity in the received signals because the received direct arrival component, for example, will contain more than one wavelength.
- the present invention is directed to a method for determining a direction of a direct arrival path between a receiver and a transmitter in a multipath environment by determining a transmitter heading relative to the receiver as proportional to a frequency offset of the direct path signal component relative to a multipath pedestal, an absolute velocity of the transmitter as proportional to a width of the multipath pedestal, a relative velocity between the transmitter and the receiver as proportional to a magnitude and a direction of doppler shift of the direct arrival component of the received signal relative to the doppler pedestal, and an amplitude of the multipath pedestal as proportional to a number and magnitude of scatterers in the multipath environment.
- the method is applied for continuous wave and modulated signals, for stationary and moving transmitters, for tracking and mapping transmitter paths, and for navigational purposes for locating a position of a receiver.
- FIG. 1A illustrates the effect of scatters and the creation of a Doppler pedestal of received components by scatterers
- FIGS. 1B and 1C illustrate the creation of a doppler pedestal in a received signal in a multipath environment when the transmitter and receiver are in relative motion
- FIG. 2 is an illustration of a direction finding array according to the present invention
- FIG. 3 is a diagrammatic illustration of a direction finding system of the present invention.
- FIG. 4 illustrate the effects of antenna spacing dependent correlation and decorrelation on the pedestal spectrum of received multipath signals
- FIG. 5 is an illustration of a mobile direction finding system of the present invention.
- FIG. 6 is an illustration of direction finding and transmitter location when the receiver is in motion
- FIGS. 7A, 7B and 7 C are illustrative diagrams of direction finding and transmitter location when the receiver and transmitter are in relative motion
- FIGS. 8A and 8B illustrate the a received signal in a multipath environment with relative motion between the transmitter and receiver
- FIG. 9 illustrates the presence of a multipath pedestal and a slewed direct arrival component in a signal received in a multipath environment from a moving transmitter, and the relationship of pedestal width and direct arrival slew to the velocity and relative bearing of the transmitter;
- FIG. 10 is an illustration of transmitter velocity versus measured ground speed as obtained from a received signal
- FIGS. 11A-11E illustrate the relationship between the multipath pedestal and slewed direct arrival component of a received signal for various transmitter paths
- FIG. 12 is a diagrammatic representation of a direction finding and tracking system for a moving transmitter in a multipath environment
- FIGS. 13A, 13B and 13 C illustrate the recovery of a CW signal with multipath pedestal and slewed direct arrival components from a modulated signal
- FIG. 14 is an illustration of navigational location of a receiver in a multipath environment.
- multipath sources 10 typically appear to surround the receiving unit 12 and to have effectively random radiation patterns 14 and arise from the reflection or refraction of a transmitted signal 16 from a transmitter 18 by “scatterers” 20 , which may be any element of the environment capable of reflecting the original signal or of refracting the original signal around themselves.
- conventional direction finding systems typically employ two or more DF (direction finding) receiving antennas 12 A and 12 B spaced apart from one another along a “baseline” 12 C and compare the amplitudes or phases of the signals received at the DF antennas 12 A and 12 B to determine the direction to the transmitter 18 .
- This method is, however, historically subject to systemic errors for a number of reasons. For example, if the DF antennas 12 A and 12 B are spaced too closely there will be correlation between the multipath components 22 M of the received signal 22 R, and between the multipath components 22 M and the direct arrival component 22 D, resulting in an induced multipath bias error that cannot be “washed out” even by time integration of the received signal components.
- the DF antennas 12 A and 12 B are spaced too far apart, such as more than one wavelength apart, the multipath components 22 M and direct arrival components 22 D will be decorrelated, but there will be phase ambiguity in the received signals because the received direct arrival component 22 D, for example, will contain more than one wavelength.
- the accuracy of identifying a true direction line 26 to a transmitting source can be significantly improved when there is relative motion between the transmitter 18 and the receiver 12 , or between either the transmitter 18 or receiver 12 and the scatterers 22 .
- the received signal 22 R is comprised of a direct arrival component 22 D, that is, a component arriving along a direct path from the transmitter to the receiver, and a plurality of multipath components 22 M arriving at the receiver 12 from the scatters along reflective and refractive paths.
- the received direct arrival component 22 D will have a Doppler shift that is determined by the relative speed and heading of the transmitter 18 and the receiver 12 while the multipath components 22 M will have a range of Doppler shifts with the Doppler shift of each multipath component 22 M being determined by the path followed between the transmitter 18 and the receiver 12 .
- the received signal 22 R will thereby be comprised of the direct arrival component 22 D having a Doppler shift determined by the relative speed and heading of the transmitter 18 and receiver 12 accompanied by, or surrounded by, a Doppler “pedestal” 22 P comprised of a plurality of multipath signals 22 M.
- the width of pedestal 22 P is twice the sum of the receiver and transmitter absolute (not relative) speeds divided by the wavelength of the signal of interest.
- the power or amplitude of the multipath components 22 M are thereby diluted by the Doppler spreading of the multipath components 22 M while the power or amplitude of the direct component 22 D is not diluted because the direct arrival component follows the direct path.
- the Doppler dilution of the amplitude or power of the multipath components 22 M relative to the direct arrival component 22 D thereby effectively provides a processing gain that can improve the accuracy of the DF result by raising the amplitude of the direct arrival component 22 D above that of the multipath components 22 M.
- the method of the present invention requires relative motion of the transmitter 18 or the receiver 12 or both, or between either or both and the scatterers 20 , because when there is no motion of either the transmitter or the receiver there is no “spreading” or “dilution” of the multipath components 22 M and the multipath pedestal 22 P collapses on top of the direct arrival component 22 D, so that the processing advantage is lost.
- the method and apparatus of the present invention employs a moving array of two or more relatively closely spaced DF antennas 12 A and 12 B and a more distantly spaced reference antenna 12 R.
- DF antennas 12 A and 12 B are spaced sufficiently close to eliminate phase ambiguity at the highest frequency of interest, recognizing that the multipath signal components 22 M received by DF antennas 12 A and 12 B will most probably by correlated in the frequency ranges of interest.
- Reference antenna 12 R is spaced apart from DF antennas 12 A and 12 B by a distance sufficient that, at the frequencies of interest, the multipath components 22 M received by reference antenna 12 R are decorrelated with respect to the direct arrival signal components 22 D received by correlated antennas 12 A and 12 B. In this regard, it must be recognized that there will most probably be phase ambiguity between the signal components 22 D and 22 M received by reference 12 R antenna with respect to the signal components 22 D and 22 M received by DF antennas 12 A and 12 B in the frequency range of interest.
- FIG. 3 The method and apparatus for determining the desired direction finding baseline according to the present invention is illustrated in FIG. 3 .
- DF antennas 12 A and 12 B are sufficiently close to eliminate phase ambiguity at the highest frequency of interest and reference antenna 12 R is spaced apart from DF antennas 12 A and 12 B by a distance sufficient that, at the frequencies of interest, the multipath components 22 M received by reference antenna 12 R are decorrelated with respect to the direct arrival signal components 22 D received by correlated antennas 12 A and 12 B.
- the corresponding receiver or receivers may be implemented as separate receivers for the three antennas, as one receiver for DF antennas 12 A and 12 B and a second receiver for reference antenna 12 R, or, as shown, as a single receiver shared among the three antennas.
- step 24 A performed by a comparator 12 a , the direct and multipath received signal components 22 DR and 22 MR of received signal 22 RR at reference antenna 12 R, are compared in phase with, that is, subtracted from, the direct and multipath received signal components 22 DA and 22 MA of received signal 22 RA at receiving antenna 12 A, giving the result 22 RA- 22 RR, or ( 22 DA+ 22 MA) ⁇ ( 22 RD+ 22 RM).
- step [ 24 B] performed by a comparator 12 b , the received signal components 22 DR and 22 MR of received signal 22 RR at reference antenna 12 R are compared in phase with, that is, subtracted from, the received signal components 22 DB and 22 MB of received signal 22 RB at DF antenna 12 B, giving the result 22 RB- 22 RR, or ( 22 DB+ 22 MB) ⁇ ( 22 DR+ 22 MR).
- a step [ 24 C] the comparison between DF antenna 12 A and reference antenna 12 R and the comparison between DF antenna 12 B and reference antenna 12 R from steps 24 A and 24 B are then differenced, that is, subtracted, in a step 24 C performed by a comparator 24 c to give the result 22 DA- 22 DB, or [( 22 DA+ 22 MA) ⁇ ( 22 RD+ 22 RM)] ⁇ [( 22 DB+ 22 MB) ⁇ ( 22 DR+ 22 MR)], which yields baseline phase angle measurement 26 P indicating the direct line 26 to the transmitting source 18 .
- the reference antenna 12 R received signal components 22 DR and 22 MR are common to the two comparisons so that the phases of the reference antenna 12 R received signal components 22 DR and 22 MR will cancel out.
- the reference antenna 12 R received signal components are decorrelated with received signal components 22 DA and 22 MA at DF antenna 12 A and with respect to received signal components and 22 MB at DF antenna 12 B, the multipath signal components 22 M will “wash out”, as illustrated in FIG. 4 .
- the comparisons and differencing will thereby provide, as a result, the desired direction finding baseline phase angle measurement 26 P indicating the direct line 26 to the transmitting source 18 .
- FIG. 5 therein is illustrated an exemplary embodiment of the correlated DF antennas 12 A and 12 B and reference antenna 12 R of the present invention wherein the three antennas are mounted onto a ground mobile vehicle 28 .
- the two correlated DF antennas 12 A and 12 B comprise a direction finding array, that may be concealed or disguised, and reference antenna 12 R may appear as no more than a conventional whip antenna.
- the frequency band to be scanned may include a relatively wide frequency band of interest, and, even in the case of a relatively narrow frequency band, will normally include the anticipated frequency width of the Doppler pedestal.
- the frequency band of interest is preferably scanned at least the Nyquist rate for the pedestal width in order to avoid fold over (aliasing) of the multipath energy.
- the presence or absence of a direct arrival component can be detected by determining whether there is a strong spectral line, that is, a strong direct signal component 22 D, protruding from the Doppler pedestal 22 P. This can be used to reject futher processing of any signals not in direct line of sight of the receiver by amplitude discrimination.
- the direct arrival component maps to the “DC” component of the pedestal, that is, the stable mid-band component of the received signal pedestal, and the angle of this DC component is the desired phase measurement.
- the intensity of the multipath can be determined by summing the power in the other, that is, non-direct arrival, bins of the Doppler pedestal, which will represent the total multipath component power, and comparing the total multipath component power to the DC level. According to Parseval's theorem, this measurement can be made in the time domain without having to actually compute the Doppler spectrum.
- FIG. 6 illustrates direction finding for the above described method of the present invention when the transmitter 18 is stationary.
- receiver 12 traverses a path 30 relative to a transmitter 18 that is in a fixed location 18 L and determines the bearing angles 32 a and 32 b of direction transmission paths 32 A and 32 B to the transmitter 18 relative to path 30 from points 30 A and 30 B of the path 30 and the location of the intersection of direct transmission paths 32 A and 32 B at location 18 L identifies the location of the transmitter 18 .
- FIG. 7A illustrates the case when transmitter 18 is moving along a transmitter path 18 P that extends in the same general direction as receiver path 30 .
- transmitter 18 is at a first location 18 LA along path 18 P and, when the direct transmission path 32 B is determined at point 30 B along path 18 P, transmitter 18 has moved to a location 18 LB along path 18 P.
- transmitter 18 was mounted on a vehicle and transmitted a CW signal at 100 mw at a frequency of 432.075 MHz from a 5 ⁇ 8th magnetic mount vertical rod antenna on the roof of the vehicle while the vehicle traveled a path 18 P from a first location in Nashua, N.H. to a second location n Manchester, N.H.
- the receiver 12 was mounted on an aircraft that flew a path 30 enclosing a major part but not all of the vehicle path 18 P.
- the resulting “sheaf” 32 S of direction finding bearings indicated that the transmitter 18 was at a false location 18 LF that appeared to be fixed and that was not only many miles from the path 18 P, but was not even at any point along the path 18 P.
- FIG. 8A illustrates data recorded during the experiment and is a frequency/time plot/signal amplitude plot of the received signal during a portion of the experiment.
- the multipath pedestal 22 P resulting from motion of the transmitter 18 and the receiver 12 is present throughout the entire period during which the transmitter 18 or the receiver 12 , or both, are in motion.
- FIG. 8A illustrates data recorded during the experiment and is a frequency/time plot/signal amplitude plot of the received signal during a portion of the experiment.
- 8B is a time/bandwidth/signal amplitude plot of the processed data and illustrates that both the multipath pedestal 22 P and the direct arrival signal component 22 D are likewise apparent throughout the period of the experiment, and that at least in this instance the multipath pedestal 22 P has an average amplitude approximately ⁇ 20 dB below that of the direct arrival signal component 22 D.
- FIG. 9 therein is shown an exemplary frequency/amplitude plot of received signal 22 R under the above described conditions.
- the width W of the multipath pedestal 22 P is determined by the absolute velocity of the transmitter 18 and that the height H of the direct arrival signal component 22 D is proportional to the “roughness” of the terrain in which the transmitter 18 is located.
- the frequency offset 8 of the direct arrival component 22 D within the multipath pedestal 22 P is directly proportional to the transmitter 18 heading relative to the receiver 12 .
- the magnitude and direction of the Doppler shift of the direct arrival component 22 D is proportional to the relative velocity between the transmitter 18 and receiver 12 and thereby contains information pertaining, for example, to whether the distance between the transmitter 18 and receiver 12 is increasing or decreasing and the speed with which the distance is increasing or decreasing.
- FIG. 10 illustrates the degree of correlation between the physical ground speed and relative heading of the vehicle and the vehicle speed calculated from the Doppler shift/relative bearing information extracted from the received signal 22 R for the experiment described above; that is, for a transmitter bearing vehicle traveling from Nashua, N.H. to downtown Manchester, N.H. and a receiving bearing aircraft circling the Manchester area.
- FIGS. 11A through 11F wherein FIG. 11A is a frequency/time plot of the received signal 22 R while the transmitter 18 traversed a known exit ramp along the path 18 P between Nashua, N.H. and Manchester, N.H. and FIG. 11B is a vertical photograph/map of the exit ramp. It can been seen from a comparison of FIGS.
- the frequency/time plot of the direct arrival component 22 D and pedestal 22 P of received signal 22 R contains a representation of the actual physical path followed by the transmitter 18 , that is, of the path traversed by the vehicle bearing the transmitter 18 , with the path being represented in terms of the speed and relative heading of the vehicle.
- FIG. 11C is a diagrammatic representation of a known cloverleaf intersection along the path 18 P while FIG. 11D is a set of calculated Doppler shift/time plots of a received signal 22 R from a transmitter 18 traversing each of the ramps of the exemplary cloverleaf intersection at a predetermined speed or speed range wherein each plot is calculated for a corresponding ramp.
- FIG. 11E is a frequency/time plot of an actual received signal 22 R from a transmitter 18 physically traversing one of the ramps of the cloverleaf intersection.
- the pedestal width W and frequency offset ⁇ of the direct arrival component 22 D of the received signal 22 R represents the speed and relative bearing of the vehicle over the indicated time period in which the vehicle is traversing one of the ramps.
- a comparison of the frequency/time plot of the actual received signal 22 R with the predicted Doppler shift/time plots for the four ramps of the cloverleaf intersection shows a correspondence with one of the four ramps, thereby allowing identification of the specific cloverleaf ramp taken by the transmitter 18 by comparison of the predicted and actual characteristics of the received signal 22 R.
- the speed and relative bearing information that can be extracted from a received signal 22 R from a moving transmitter will allow the reconstruction and mapping of the path traversed by the transmitter.
- the above discussed direction finding method and apparatus for finding and tracking the location of a moving transmitter 18 is illustrated in FIG. 12 for the example illustrated in FIG. 7C wherein a transmitter 18 mounted on a vehicle traverses path 18 P from Nashua, N.H. to Manchester, N.H. while the receiver 12 is mounted on an aircraft flying along path 30 .
- the method and apparatus of the present invention requires only a single receiving antenna in the case when the transmitter is in motion, rather than the two DF antennas and reference antenna used when the transmitter is stationary.
- the receiver 12 is represented as capturing received signal 22 R at each of the sequence of receiver path points 12 a - 12 i along path 30 and the corresponding locations of the transmitter 18 along path 18 P are indicated as the sequence of transmitter path points 18 a - 18 i .
- receiver 12 captures the received signal 22 R from transmitter 18 and provides an output 34 A signal that includes information representing the multipath pedestal 22 P and the direct arrival component 22 D of signal 22 R resulting at the corresponding one of receiver path points 12 a - 12 i by the transmission from the corresponding one of transmitter path points 18 a - 18 i .
- a pedestal width/frequency offset ⁇ processor 34 B extracts the pedestal width W information and frequency offset ⁇ information from the current output of receiver 12 and generates, for each receiver path point 12 a - 12 i and corresponding transmitter path point 18 a - 18 i , an output 34 C representing the corresponding absolute velocity Va-Vi of the transmitter 18 and relative bearing Ba-Bi of the transmitter 18 relative to the receiver 12 at that point 12 a - 12 i / 18 a - 18 i.
- the outputs 34 C may then be used to construct a track map 34 D representing the velocity and relative bearing of the transmitter at each point 12 a - 12 i / 18 a - 18 i and, if the location of the receiver 12 is known at each of receiver path points 12 a - 12 i , the track map 34 D may represent the absolute velocity and physical bearing of the transmitter 18 at each of points 18 a - 18 i .
- the actual distance between receiver 12 and transmitter 18 at each of these points has not been determined. As a consequence, there may be some ambiguity in the transmitter track mapping at this point.
- the actual geophysical path traversed by transmitter 18 may be resolved, however, in a number of ways.
- the local bearing of the transmitter 18 at each point 18 a - 18 i can be determined from the corresponding known relative bearing between the transmitter 18 and receiver 12 , which has been determined as described above, and the known receiver 12 bearing for each corresponding receiver path point 12 a - 12 i .
- the absolute speed and local bearing of the transmitter 18 at each transmitter path point 18 a - 18 i could then be used to construct a dead reckoning plot of the path of the transmitter 18 from a known starting point, or be back tracking from a known ending point.
- An alternate method using cross bearing from a second receiver would require additional resources, but would be simpler and more direct.
- This method would use a second receiver 12 that is generally synchronized with the first receiver 12 as regards their capture of their respective received signals 22 R, thereby providing obtaining cross bearings for each transmitter path point 18 a - 18 i.
- FIG. 12 An yet further method for determining the actual geophysical location of each transmitter path point 18 a - 18 i is illustrated in FIG. 12 and is by comparison of the sequence of relative bearing/velocity captures 34 C or 34 D with a map or maps 34 E of the possible transmitter paths 18 P.
- the map or maps 34 E could be geophysical maps 34 EG and a matching processor 34 F would compare the bearing/velocity captures 34 C or 34 D with the possible transmitter paths 18 P represented in the geophysical map or maps 34 EG, attempting to find a best match between the relative bearing/velocity captures and the geophysical paths 18 P.
- predicted received signal characteristic frequency/time plots 34 EP would be generated for a range of possible or probable route segments or points and possible or probable speed ranges for an area of interest, such as specific sections of road or intersections through the area.
- the predicted received signal characteristic plots 34 EP could, for example, contain information in the form of possible pedestal width W/frequency offset ⁇ plots for selected points or segments along possible transmitter paths 18 P.
- the pedestal width W/frequency offset ⁇ extracted from the received signals 22 R would then be compared with the stored pedestal width W/frequency offset ⁇ plots 34 EP to determine the best matches, if any, to identify the route segments or points best corresponding with the received data and thereby allowing identification and reconstruction of the actual path traversed by the transmitter.
- the generation of predicted received signal characteristic plots and the identification and tracing of traversed path segments in a given area will be effected by the range and complexity of the possible paths in the area. That is, the generation of predicted received signal characteristic plots for established roads and highways in a given area is relatively straightforward as the locations and paths followed by the roads and highways can be ascertained from, for example, topographical maps or satellite or aerial photographs or maps. The situation becomes more complex when, for example, it is necessary to consider “off road” paths and, in this instance, the generation of predicted received signal characteristic plots will depend upon the complexities of the terrain, such as how many practicable paths are possible in the terrain.
- the number of possible paths may be very limited in some terrain, such as in large areas of New England or the canyon country of the south-west, but could become very large for relatively level terrain with few obstacles. In the latter case, however, direct tracking of a vehicle as described above, rather than tracking by matching to predetermined plots, would be available and may in fact be preferable.
- FIGS. 13A and 13B are illustrations of exemplary QPSK signals wherein FIG. 13A is an amplitude/frequency representation of a QPSK signal with white Gaussian noise and multipath effects.
- FIG. 13B is a “constellation” map of a QPSK signal with white Gaussian noise and multipath effects.
- a constellation map for a QPSK signal divides the signal space into four quadrants separated by diagonal “decision lines” wherein the crossing point of the decision lines represents the carrier frequency of the QPSK signal.
- Each quadrant represents one of the four possible phase angle encodings of a corresponding information symbol and the decision lines represent the decision thresholds determining to which quadrant a received symbol represented by a phase angle is assigned.
- FIG. 13C is an illustrative frequency/amplitude plot of a received signal 22 R that has been received in QPSK format and that has been recovered from the QPSK format by conventional demodulation/remodulation processing as illustrated in FIG. 13D to effectively convert the received modulated signal into a continuous wave signal, thereby recovering the multipath and direct arrival components.
- receiver 12 receives signal 22 R, which is in QPSK format.
- Receiver 12 provides the received QPSK format signal to a demodulator/modulator 36 , which demodulates and re-modulates the QPSK signal to generate a recovered signal 22 Rr having the pedestal 22 P and direct arrival component 22 D characteristics illustrated in FIG. 13C .
- the resulting recovered received signal 22 Rr contains all of the characteristics previously illustrated and discussed with regard to FIG. 9 . That is, the recovered signal 22 R includes a multipath pedestal 22 P having a frequency width W determined by the absolute velocity of the transmitter 18 and a direct arrival component 22 D having height H proportional to the “roughness” of the terrain in which the transmitter 18 is located and a frequency offset ⁇ within the multipath pedestal 22 P proportional to the transmitter 18 heading relative to the receiver 12 .
- the recovered signal 22 Rr is then processed by a pedestal width/frequency offset ⁇ processor 34 B as shown in FIG. 12 to provide the desired pedestal width W and frequency offset ⁇ outputs, which may be employed as described herein above.
- the method and apparatus of the present invention may be implemented for modulated signals as well as continuous wave signals by suitable processing of the received signal to effectively convert the received modulated signal into a continuous wave signal to recover the multipath component 22 M and direct arrival component 22 D of the received signal.
- the location of the receiver 12 may be determined so long as there is relative motion between the receiver 12 and a pair of transmitters 18 having known locations and transmitting CW or modulated signals and so long as the receiver 12 is in motion along a known heading.
- the received signal 22 R from each transmitter 18 will thereby either contain a pedestal component 22 P and a direct arrival component 22 D, or a pedestal component 22 P and a direct arrival component 22 D can be recovered from the received signal 22 R by demodulation/remodulation methods as described above.
- the location of the direct arrival component 22 D within the Doppler pedestal 22 P of each received signal 22 R will then indicate the relative bearing of the corresponding transmitter 18 with respect to the moving receiver 12 , and the intersection of the headings will indicate the current location of the receiver 12 .
Abstract
Description
- The present invention relates to the location of stationary and mobile transmitters by direction finding from mobile vehicles and, in particular, in a multipath environment.
- There are many circumstances wherein it is necessary or desirable to determine the geographic location of an emitter of electromagnetic radiation, such as a radar system, a communications facility or device or an emergency beacon or transmitter. Typical applications may include, for example, military signal intelligence (SIGINT) and electronic intelligence (ELINT) operations for locating radar or communications facilities, and air, land and sea rescue operations wherein it is necessary to locate an emergency beacon or transmitter, such as used in aircraft and vessels, or communications devices ranging from conventional or emergency radio devices to cell phones.
- Such applications and operations are characterized by common requirements that are, in turn, imposed by general, common characteristics of the target emitters to be located and the situations or circumstances under which the target emitters are to be located. For example, the signal transmitted by a target emitter may be of relatively low power, as in the case of emergency beacons or emergency radios, or may be masked, distorted or effectively reduced by terrain or weather conditions, and such conditions may be intentionally imposed in, for example, military or otherwise hostile situations. In addition, the time available or permissible for locating a target emitter may be limited in both military and civil situations, that is, and for example, in military counter-measures operations or in search and rescue operations, and the resources available for target emitter location may be limited.
- As such, it is generally necessary or desirable for a system for locating target emitters to be mobile, that is, to be readily transportable into the general geographical location of a target emitter on an aircraft, vehicle or vessel, both to bring the locator system into range of the target emitter and to allow the locator system to search as large an area as possible in the minimum time. It is also desirable that a locator system be transported and employed in and from a single platform, whether an aircraft, vessel or vehicle, as the use of a single platform reduces the system cost, reduces demand on frequently limited resources and allows a greater area or number of areas to be searched when multiple platforms are available. A single platform system also eliminates the complexity and time delays inherent in deploying and coordinating multiple cooperatively operating platforms.
- The locator system must be capable of identifying the geographic location of a target emitter with the greatest possible accuracy as insufficient accuracy in locating a target emitter may render counter-measures ineffective in military situations and may unacceptably delay locating or reaching the target emitter in civil situations, such as search and rescue operations, particularly in difficult terrain or weather conditions. In addition, the locator system should be capable of locating as wide a range of target emitter types as possible, and correspondingly over as wide a range of the electromagnetic spectrum as possible, to allow a given locator system to be employed in as wide a range of applications and situations as possible.
- There are a number of factors that determine and limit the characteristics and capabilities of an emitter location system, and in particular a single platform, mobile emitter location system, are numerous and inter-related. For example, current methods for single platform emitter location are based upon determining multiple direction finding (DF) bearings, often referred to as DF “cuts”, to the target emitter at points along a path traversed by the locator platform, such as the flight path of an aircraft. Each “cut” is an attempt to determine the direction of the emitter relative to the locator platform at the point the “cut” is taken by using an amplitude or phase detecting directional antenna and receiver array to determine the direction of the strongest signal component or the phase gradient, that is, the direction of propagation, of the wavefront of the received signal. Successive DF cuts may be used to determine a Line of Bearing (LOB) “fan” of DF cuts, with the location of the target emitter being taken as the point of intersection of the DF bearings forming the LOB fan.
- These method of the prior art are, however, subject to significant limitations and problems. For example, signal propagation factors between the emitter and the locator system path at various points, such as variations in propagation conditions, local multipath distortions, multiple propagation paths and reflections, will result in significant errors in the measured gradients of the wavefront and this significant errors in the measured bearings between the locator system and the target emitter.
- One of if not the most significant problem in the direction finding methods of the prior art is that of multipathing, that is, the tendency for a received signal to appear to arrive from multiple sources separated from the true source. This phenomenon is well known to communications engineers, as evidenced, for example, by R. H. Clarke: “A Statistical Theory for Mobile Radio Communications,” Bell System Technical Journal, July 1968, 47, pp. 957-1000).
- In brief, multipath sources typically appear to surround the receiving unit and to have effectively random radiation patterns and arise from the reflection or refraction of the transmitted signal by “scatterers”, which may be any element of the environment capable of reflecting the original signal or of refracting the original signal around themselves.
- Conventional direction finding systems typically employ two or more receiving antennas spaced apart from one another along a “baseline” and compare the amplitudes or phases of the signals received at the antennas to determine the direction to the transmitter. This method is, however, historically subject to systemic errors for a number of reasons. For example, if the antennas are spaced too closely there will be correlation between the multipath components of the received signal, and between the multipath components and the direct arrival component, resulting in an induced multipath bias error that cannot be “washed out” even by time integration of the received signal components. If, however, the antennas are spaced too far apart, such as more than one wavelength apart, the multipath and direct arrival components will be decorrelated, but there will be phase ambiguity in the received signals because the received direct arrival component, for example, will contain more than one wavelength.
- The present invention is directed to a method for determining a direction of a direct arrival path between a receiver and a transmitter in a multipath environment by determining a transmitter heading relative to the receiver as proportional to a frequency offset of the direct path signal component relative to a multipath pedestal, an absolute velocity of the transmitter as proportional to a width of the multipath pedestal, a relative velocity between the transmitter and the receiver as proportional to a magnitude and a direction of doppler shift of the direct arrival component of the received signal relative to the doppler pedestal, and an amplitude of the multipath pedestal as proportional to a number and magnitude of scatterers in the multipath environment.
- The method is applied for continuous wave and modulated signals, for stationary and moving transmitters, for tracking and mapping transmitter paths, and for navigational purposes for locating a position of a receiver.
- The above discussed aspects of the prior art and the following discussed aspects of the present invention are illustrated in the figures, wherein:
-
FIG. 1A illustrates the effect of scatters and the creation of a Doppler pedestal of received components by scatterers; -
FIGS. 1B and 1C illustrate the creation of a doppler pedestal in a received signal in a multipath environment when the transmitter and receiver are in relative motion; -
FIG. 2 is an illustration of a direction finding array according to the present invention; -
FIG. 3 is a diagrammatic illustration of a direction finding system of the present invention; -
FIG. 4 illustrate the effects of antenna spacing dependent correlation and decorrelation on the pedestal spectrum of received multipath signals; -
FIG. 5 is an illustration of a mobile direction finding system of the present invention; -
FIG. 6 is an illustration of direction finding and transmitter location when the receiver is in motion; -
FIGS. 7A, 7B and 7C are illustrative diagrams of direction finding and transmitter location when the receiver and transmitter are in relative motion; -
FIGS. 8A and 8B illustrate the a received signal in a multipath environment with relative motion between the transmitter and receiver; -
FIG. 9 illustrates the presence of a multipath pedestal and a slewed direct arrival component in a signal received in a multipath environment from a moving transmitter, and the relationship of pedestal width and direct arrival slew to the velocity and relative bearing of the transmitter; -
FIG. 10 is an illustration of transmitter velocity versus measured ground speed as obtained from a received signal; -
FIGS. 11A-11E illustrate the relationship between the multipath pedestal and slewed direct arrival component of a received signal for various transmitter paths; -
FIG. 12 is a diagrammatic representation of a direction finding and tracking system for a moving transmitter in a multipath environment; -
FIGS. 13A, 13B and 13C illustrate the recovery of a CW signal with multipath pedestal and slewed direct arrival components from a modulated signal; and, -
FIG. 14 is an illustration of navigational location of a receiver in a multipath environment. - First considering multipath and the problems arising from multipath as illustrated diagrammatically in
FIG. 1A ,multipath sources 10 typically appear to surround the receivingunit 12 and to have effectivelyrandom radiation patterns 14 and arise from the reflection or refraction of a transmitted signal 16 from atransmitter 18 by “scatterers” 20, which may be any element of the environment capable of reflecting the original signal or of refracting the original signal around themselves. - As illustrated in
FIG. 1A , conventional direction finding systems typically employ two or more DF (direction finding) receivingantennas DF antennas transmitter 18. This method is, however, historically subject to systemic errors for a number of reasons. For example, if theDF antennas multipath components 22M of the receivedsignal 22R, and between themultipath components 22M and thedirect arrival component 22D, resulting in an induced multipath bias error that cannot be “washed out” even by time integration of the received signal components. If, however, theDF antennas multipath components 22M anddirect arrival components 22D will be decorrelated, but there will be phase ambiguity in the received signals because the receiveddirect arrival component 22D, for example, will contain more than one wavelength. - According to the present invention, and according to the measurement configurations and processing algorithms of the present invention, the accuracy of identifying a true direction line 26 to a transmitting source can be significantly improved when there is relative motion between the
transmitter 18 and thereceiver 12, or between either thetransmitter 18 orreceiver 12 and the scatterers 22. - When there is motion of the
transmitter 18 or thereceiver 12 or both, or between either or both and thescatterers 20, the receivedsignal 22R is comprised of adirect arrival component 22D, that is, a component arriving along a direct path from the transmitter to the receiver, and a plurality ofmultipath components 22M arriving at thereceiver 12 from the scatters along reflective and refractive paths. The receiveddirect arrival component 22D will have a Doppler shift that is determined by the relative speed and heading of thetransmitter 18 and thereceiver 12 while themultipath components 22M will have a range of Doppler shifts with the Doppler shift of eachmultipath component 22M being determined by the path followed between thetransmitter 18 and thereceiver 12. In summary, therefore, and as illustrated inFIGS. 1B and 1C , the receivedsignal 22R will thereby be comprised of thedirect arrival component 22D having a Doppler shift determined by the relative speed and heading of thetransmitter 18 andreceiver 12 accompanied by, or surrounded by, a Doppler “pedestal” 22P comprised of a plurality ofmultipath signals 22M. The width ofpedestal 22P is twice the sum of the receiver and transmitter absolute (not relative) speeds divided by the wavelength of the signal of interest. - As may be seen from
FIGS. 1B and 1C , the power or amplitude of themultipath components 22M are thereby diluted by the Doppler spreading of themultipath components 22M while the power or amplitude of thedirect component 22D is not diluted because the direct arrival component follows the direct path. The Doppler dilution of the amplitude or power of themultipath components 22M relative to thedirect arrival component 22D thereby effectively provides a processing gain that can improve the accuracy of the DF result by raising the amplitude of thedirect arrival component 22D above that of themultipath components 22M. It must be noted, however, that the method of the present invention requires relative motion of thetransmitter 18 or thereceiver 12 or both, or between either or both and thescatterers 20, because when there is no motion of either the transmitter or the receiver there is no “spreading” or “dilution” of themultipath components 22M and themultipath pedestal 22P collapses on top of thedirect arrival component 22D, so that the processing advantage is lost. - A. Direction Finding For A Stationary Transmitter In A Multipath Environment
- First considering implementations of the present invention for those cases wherein the
transmitter 18 is stationary, and according to the present invention as illustrated inFIG. 2 , the method and apparatus of the present invention employs a moving array of two or more relatively closely spacedDF antennas reference antenna 12R. For example,DF antennas multipath signal components 22M received byDF antennas -
Reference antenna 12R, however, is spaced apart fromDF antennas multipath components 22M received byreference antenna 12R are decorrelated with respect to the directarrival signal components 22D received by correlatedantennas signal components reference 12R antenna with respect to thesignal components DF antennas - The method and apparatus for determining the desired direction finding baseline according to the present invention is illustrated in
FIG. 3 . As illustrated therein,DF antennas reference antenna 12R is spaced apart fromDF antennas multipath components 22M received byreference antenna 12R are decorrelated with respect to the directarrival signal components 22D received by correlatedantennas receiver 12, may be implemented as separate receivers for the three antennas, as one receiver forDF antennas reference antenna 12R, or, as shown, as a single receiver shared among the three antennas. - In a [
step 24A], performed by acomparator 12 a, the direct and multipath received signal components 22DR and 22MR of received signal 22RR atreference antenna 12R, are compared in phase with, that is, subtracted from, the direct and multipath received signal components 22DA and 22MA of received signal 22RA at receivingantenna 12A, giving the result 22RA-22RR, or (22DA+22MA)−(22RD+22RM). At the same time, in step [24B], performed by acomparator 12 b, the received signal components 22DR and 22MR of received signal 22RR atreference antenna 12R are compared in phase with, that is, subtracted from, the received signal components 22DB and 22MB of received signal 22RB atDF antenna 12B, giving the result 22RB-22RR, or (22DB+22MB)−(22DR+22MR). Then, in a step [24C], the comparison betweenDF antenna 12A andreference antenna 12R and the comparison betweenDF antenna 12B andreference antenna 12R fromsteps step 24C performed by acomparator 24 c to give the result 22DA-22DB, or [(22DA+22MA)−(22RD+22RM)]−[(22DB+22MB)−(22DR+22MR)], which yields baselinephase angle measurement 26P indicating the direct line 26 to the transmittingsource 18. - It will therefore be seen that the
reference antenna 12R received signal components 22DR and 22MR are common to the two comparisons so that the phases of thereference antenna 12R received signal components 22DR and 22MR will cancel out. In addition, and because thereference antenna 12R received signal components are decorrelated with received signal components 22DA and 22MA atDF antenna 12A and with respect to received signal components and 22MB atDF antenna 12B, themultipath signal components 22M will “wash out”, as illustrated inFIG. 4 . The comparisons and differencing will thereby provide, as a result, the desired direction finding baselinephase angle measurement 26P indicating the direct line 26 to the transmittingsource 18. - Referring to
FIG. 5 , therein is illustrated an exemplary embodiment of the correlatedDF antennas reference antenna 12R of the present invention wherein the three antennas are mounted onto a groundmobile vehicle 28. As indicated, the two correlatedDF antennas reference antenna 12R may appear as no more than a conventional whip antenna. - Other factors that should be noted in implementing the invention include, for example, the ability to repeatedly scan a given frequency band in order to detect newly appearing transmitters. In this regard, it should be noted that the frequency band to be scanned may include a relatively wide frequency band of interest, and, even in the case of a relatively narrow frequency band, will normally include the anticipated frequency width of the Doppler pedestal. In such instances, the frequency band of interest is preferably scanned at least the Nyquist rate for the pedestal width in order to avoid fold over (aliasing) of the multipath energy.
- Also, the presence or absence of a direct arrival component can be detected by determining whether there is a strong spectral line, that is, a strong
direct signal component 22D, protruding from theDoppler pedestal 22P. This can be used to reject futher processing of any signals not in direct line of sight of the receiver by amplitude discrimination. In addition, and when all of the receiving antennas move as a integral unit, such as when all of the receiving antennas are mounted on the same vehicle, the direct arrival component maps to the “DC” component of the pedestal, that is, the stable mid-band component of the received signal pedestal, and the angle of this DC component is the desired phase measurement. It should also be recognized that the intensity of the multipath can be determined by summing the power in the other, that is, non-direct arrival, bins of the Doppler pedestal, which will represent the total multipath component power, and comparing the total multipath component power to the DC level. According to Parseval's theorem, this measurement can be made in the time domain without having to actually compute the Doppler spectrum. - B. Direction Finding for a Moving Transmitter in a Multipath Environment
- Next considering the case of a moving
transmitter 18 in a multipath environment,FIG. 6 illustrates direction finding for the above described method of the present invention when thetransmitter 18 is stationary. In that case, as discussed above,receiver 12 traverses apath 30 relative to atransmitter 18 that is in a fixedlocation 18L and determines the bearing angles 32 a and 32 b ofdirection transmission paths transmitter 18 relative topath 30 frompoints path 30 and the location of the intersection ofdirect transmission paths location 18L identifies the location of thetransmitter 18. - Therefore considering the case when
transmitter 18 is in motion along atransmitter path 18P,FIG. 7A illustrates the case whentransmitter 18 is moving along atransmitter path 18P that extends in the same general direction asreceiver path 30. When thedirect transmission path 32A is determined atpoint 30A ofpath 30,transmitter 18 is at a first location 18LA alongpath 18P and, when thedirect transmission path 32B is determined atpoint 30B alongpath 18P,transmitter 18 has moved to a location 18LB alongpath 18P. As a result, and although the direction angles of direction transmission paths 18LA and 18LB relative to the locations ofreceiver 12 are correct at each ofpoints transmitter 18 alongpath 18P will result in the apparent intersection of the directiontransmission path vectors transmitter 18 moving in the same general direction as thereceiver 12, the false intersection point 18LF will lie on both directtransmission path vectors path 18P in the direction away fromreceiver 12. The same type of error will result in the case whentransmitter 18 is moving in a direction opposite to thereceiver 12, but as illustrated inFIG. 7B the error will be in the opposite direction; that is, thetransmitter 18 will appear to be closer toreceiver 12. - This problem is further illustrated in
FIG. 7C for an experiment whereintransmitter 18 was mounted on a vehicle and transmitted a CW signal at 100 mw at a frequency of 432.075 MHz from a ⅝th magnetic mount vertical rod antenna on the roof of the vehicle while the vehicle traveled apath 18P from a first location in Nashua, N.H. to a second location n Manchester, N.H. Thereceiver 12 was mounted on an aircraft that flew apath 30 enclosing a major part but not all of thevehicle path 18P. As shown, the resulting “sheaf” 32S of direction finding bearings indicated that thetransmitter 18 was at a false location 18LF that appeared to be fixed and that was not only many miles from thepath 18P, but was not even at any point along thepath 18P. - 1. Direction Finding and Path Mapping for a Moving Transmitter in a Multipath Environment, Continuous Wave (CW) Signal
- First considering the case of a continuous wave (CW) signal emitted by a moving transmitter, and continuing with the results obtained from the above described experiment,
FIG. 8A illustrates data recorded during the experiment and is a frequency/time plot/signal amplitude plot of the received signal during a portion of the experiment. As may be readily seen fromFIG. 8A , themultipath pedestal 22P resulting from motion of thetransmitter 18 and thereceiver 12 is present throughout the entire period during which thetransmitter 18 or thereceiver 12, or both, are in motion.FIG. 8B , in turn, is a time/bandwidth/signal amplitude plot of the processed data and illustrates that both themultipath pedestal 22P and the directarrival signal component 22D are likewise apparent throughout the period of the experiment, and that at least in this instance themultipath pedestal 22P has an average amplitude approximately −20 dB below that of the directarrival signal component 22D. - Referring now to
FIG. 9 , therein is shown an exemplary frequency/amplitude plot of receivedsignal 22R under the above described conditions. As illustrated therein, it has been found that the width W of themultipath pedestal 22P is determined by the absolute velocity of thetransmitter 18 and that the height H of the directarrival signal component 22D is proportional to the “roughness” of the terrain in which thetransmitter 18 is located. It has also been found that the frequency offset 8 of thedirect arrival component 22D within themultipath pedestal 22P, in this case relative to the lower frequency edge of themultipath pedestal 22P, is directly proportional to thetransmitter 18 heading relative to thereceiver 12. In addition, and as previously discussed, the magnitude and direction of the Doppler shift of thedirect arrival component 22D is proportional to the relative velocity between thetransmitter 18 andreceiver 12 and thereby contains information pertaining, for example, to whether the distance between thetransmitter 18 andreceiver 12 is increasing or decreasing and the speed with which the distance is increasing or decreasing. -
FIG. 10 illustrates the degree of correlation between the physical ground speed and relative heading of the vehicle and the vehicle speed calculated from the Doppler shift/relative bearing information extracted from the receivedsignal 22R for the experiment described above; that is, for a transmitter bearing vehicle traveling from Nashua, N.H. to downtown Manchester, N.H. and a receiving bearing aircraft circling the Manchester area. - It is apparent from
FIG. 10 and from the above discussions that the pedestal width W and frequency offset δ factors in a receivedsignal 22R comprised of adirect arrival component 22D and amultipath pedestal 22P provide significant information about the path traversed by thetransmitter 18 with regard to both direction and speed. The extent and degree of detail of this information is illustrated inFIGS. 11A through 11F whereinFIG. 11A is a frequency/time plot of the receivedsignal 22R while thetransmitter 18 traversed a known exit ramp along thepath 18P between Nashua, N.H. and Manchester, N.H. andFIG. 11B is a vertical photograph/map of the exit ramp. It can been seen from a comparison ofFIGS. 11A and 11B that the frequency/time plot of thedirect arrival component 22D andpedestal 22P of receivedsignal 22R contains a representation of the actual physical path followed by thetransmitter 18, that is, of the path traversed by the vehicle bearing thetransmitter 18, with the path being represented in terms of the speed and relative heading of the vehicle. - In further example,
FIG. 11C is a diagrammatic representation of a known cloverleaf intersection along thepath 18P whileFIG. 11D is a set of calculated Doppler shift/time plots of a receivedsignal 22R from atransmitter 18 traversing each of the ramps of the exemplary cloverleaf intersection at a predetermined speed or speed range wherein each plot is calculated for a corresponding ramp.FIG. 11E , in turn, is a frequency/time plot of an actual receivedsignal 22R from atransmitter 18 physically traversing one of the ramps of the cloverleaf intersection. As described above, the pedestal width W and frequency offset δ of thedirect arrival component 22D of the receivedsignal 22R represents the speed and relative bearing of the vehicle over the indicated time period in which the vehicle is traversing one of the ramps. A comparison of the frequency/time plot of the actual receivedsignal 22R with the predicted Doppler shift/time plots for the four ramps of the cloverleaf intersection shows a correspondence with one of the four ramps, thereby allowing identification of the specific cloverleaf ramp taken by thetransmitter 18 by comparison of the predicted and actual characteristics of the receivedsignal 22R. - According to the present invention, therefore, the speed and relative bearing information that can be extracted from a received
signal 22R from a moving transmitter will allow the reconstruction and mapping of the path traversed by the transmitter. The above discussed direction finding method and apparatus for finding and tracking the location of a movingtransmitter 18 is illustrated inFIG. 12 for the example illustrated inFIG. 7C wherein atransmitter 18 mounted on a vehicle traversespath 18P from Nashua, N.H. to Manchester, N.H. while thereceiver 12 is mounted on an aircraft flying alongpath 30. - At this point it must be noted that the method and apparatus of the present invention requires only a single receiving antenna in the case when the transmitter is in motion, rather than the two DF antennas and reference antenna used when the transmitter is stationary.
- In the simplified example of
FIG. 12 thereceiver 12 is represented as capturing receivedsignal 22R at each of the sequence of receiver path points 12 a-12 i alongpath 30 and the corresponding locations of thetransmitter 18 alongpath 18P are indicated as the sequence of transmitter path points 18 a-18 i. As illustrated inFIG. 12 , at each ofpoints 12a -12 i receiver 12 captures the receivedsignal 22R fromtransmitter 18 and provides anoutput 34A signal that includes information representing themultipath pedestal 22P and thedirect arrival component 22D ofsignal 22R resulting at the corresponding one of receiver path points 12 a-12 i by the transmission from the corresponding one of transmitter path points 18 a-18 i. A pedestal width/frequency offset δ processor 34B extracts the pedestal width W information and frequency offset δ information from the current output ofreceiver 12 and generates, for each receiver path point 12 a-12 i and corresponding transmitter path point 18 a-18 i, anoutput 34C representing the corresponding absolute velocity Va-Vi of thetransmitter 18 and relative bearing Ba-Bi of thetransmitter 18 relative to thereceiver 12 at thatpoint 12 a-12 i/18 a-18 i. - The
outputs 34C may then be used to construct atrack map 34D representing the velocity and relative bearing of the transmitter at eachpoint 12 a-12 i/18 a-18 i and, if the location of thereceiver 12 is known at each of receiver path points 12 a-12 i, thetrack map 34D may represent the absolute velocity and physical bearing of thetransmitter 18 at each ofpoints 18 a-18 i. In this regard, it will be noted that while the physical bearing to each of transmitter path points 18 a-18 i from the corresponding receiver path points 12 a-12 i ofreceiver 12 alongreceiver path 30 has been determined, the actual distance betweenreceiver 12 andtransmitter 18 at each of these points has not been determined. As a consequence, there may be some ambiguity in the transmitter track mapping at this point. The actual geophysical path traversed bytransmitter 18 may be resolved, however, in a number of ways. - On approach, for example, is by “dead reckoning” from a known starting or ending point of the
transmitter path 18P. That is, the local bearing of thetransmitter 18 at eachpoint 18 a-18 i can be determined from the corresponding known relative bearing between thetransmitter 18 andreceiver 12, which has been determined as described above, and the knownreceiver 12 bearing for each corresponding receiver path point 12 a-12 i. The absolute speed and local bearing of thetransmitter 18 at each transmitter path point 18 a-18 i could then be used to construct a dead reckoning plot of the path of thetransmitter 18 from a known starting point, or be back tracking from a known ending point. - An alternate method using cross bearing from a second receiver would require additional resources, but would be simpler and more direct. This method would use a
second receiver 12 that is generally synchronized with thefirst receiver 12 as regards their capture of their respective receivedsignals 22R, thereby providing obtaining cross bearings for each transmitter path point 18 a-18 i. - An yet further method for determining the actual geophysical location of each transmitter path point 18 a-18 i is illustrated in
FIG. 12 and is by comparison of the sequence of relative bearing/velocity captures 34C or 34D with a map ormaps 34E of thepossible transmitter paths 18P. In one embodiment of this method, the map or maps 34E could be geophysical maps 34EG and a matchingprocessor 34F would compare the bearing/velocity captures 34C or 34D with thepossible transmitter paths 18P represented in the geophysical map or maps 34EG, attempting to find a best match between the relative bearing/velocity captures and thegeophysical paths 18P. - In yet another implementation of the present invention predicted received signal characteristic frequency/time plots 34EP would be generated for a range of possible or probable route segments or points and possible or probable speed ranges for an area of interest, such as specific sections of road or intersections through the area. The predicted received signal characteristic plots 34EP could, for example, contain information in the form of possible pedestal width W/frequency offset δ plots for selected points or segments along
possible transmitter paths 18P. The pedestal width W/frequency offset δ extracted from the received signals 22R would then be compared with the stored pedestal width W/frequency offset δ plots 34EP to determine the best matches, if any, to identify the route segments or points best corresponding with the received data and thereby allowing identification and reconstruction of the actual path traversed by the transmitter. - It will be recognized that the generation of predicted received signal characteristic plots and the identification and tracing of traversed path segments in a given area will be effected by the range and complexity of the possible paths in the area. That is, the generation of predicted received signal characteristic plots for established roads and highways in a given area is relatively straightforward as the locations and paths followed by the roads and highways can be ascertained from, for example, topographical maps or satellite or aerial photographs or maps. The situation becomes more complex when, for example, it is necessary to consider “off road” paths and, in this instance, the generation of predicted received signal characteristic plots will depend upon the complexities of the terrain, such as how many practicable paths are possible in the terrain. The number of possible paths may be very limited in some terrain, such as in large areas of New England or the canyon country of the south-west, but could become very large for relatively level terrain with few obstacles. In the latter case, however, direct tracking of a vehicle as described above, rather than tracking by matching to predetermined plots, would be available and may in fact be preferable.
- 1. Direction Finding and Path Mapping for a Moving Transmitter in a Multipath Environment, Modulated Signal
- The above implementations of the present invention have been discussed with respect to systems wherein the
transmitter 18 is emitting a continuous wave (CW) signal. The system and method of the present invention may be implemented, however, for modulated signals, such as transmissions carrying digital or analog data. The following will therefore discuss exemplary implementations of the present invention for systems using, for example, a quadrature phase shift keyed (QPSK) signal, wherein each transmitted “symbol” is represented by one of four possible phase shift angles, thereby allowing each transmitted “symbol” to represent two binary data bits. -
FIGS. 13A and 13B are illustrations of exemplary QPSK signals whereinFIG. 13A is an amplitude/frequency representation of a QPSK signal with white Gaussian noise and multipath effects.FIG. 13B is a “constellation” map of a QPSK signal with white Gaussian noise and multipath effects. As is well understood by those of ordinary skill in the relevant arts, a constellation map for a QPSK signal divides the signal space into four quadrants separated by diagonal “decision lines” wherein the crossing point of the decision lines represents the carrier frequency of the QPSK signal. Each quadrant represents one of the four possible phase angle encodings of a corresponding information symbol and the decision lines represent the decision thresholds determining to which quadrant a received symbol represented by a phase angle is assigned. -
FIG. 13C , in turn, is an illustrative frequency/amplitude plot of a receivedsignal 22R that has been received in QPSK format and that has been recovered from the QPSK format by conventional demodulation/remodulation processing as illustrated inFIG. 13D to effectively convert the received modulated signal into a continuous wave signal, thereby recovering the multipath and direct arrival components. As indicated therein,receiver 12 receivessignal 22R, which is in QPSK format.Receiver 12 provides the received QPSK format signal to a demodulator/modulator 36, which demodulates and re-modulates the QPSK signal to generate a recovered signal 22Rr having thepedestal 22P anddirect arrival component 22D characteristics illustrated inFIG. 13C . - As shown, the resulting recovered received signal 22Rr contains all of the characteristics previously illustrated and discussed with regard to
FIG. 9 . That is, the recoveredsignal 22R includes amultipath pedestal 22P having a frequency width W determined by the absolute velocity of thetransmitter 18 and adirect arrival component 22D having height H proportional to the “roughness” of the terrain in which thetransmitter 18 is located and a frequency offset δ within themultipath pedestal 22P proportional to thetransmitter 18 heading relative to thereceiver 12. The recovered signal 22Rr is then processed by a pedestal width/frequency offset δ processor 34B as shown inFIG. 12 to provide the desired pedestal width W and frequency offset δ outputs, which may be employed as described herein above. - It is therefore apparent that the method and apparatus of the present invention may be implemented for modulated signals as well as continuous wave signals by suitable processing of the received signal to effectively convert the received modulated signal into a continuous wave signal to recover the
multipath component 22M anddirect arrival component 22D of the received signal. - C. Navigation in a Multipath Environment
- Lastly, consideration of the above discussions will show that it is possible to use the above described methods and apparatus of the present invention for navigational purposes, as illustrated in
FIG. 14 . In this instance the location of thereceiver 12 may be determined so long as there is relative motion between thereceiver 12 and a pair oftransmitters 18 having known locations and transmitting CW or modulated signals and so long as thereceiver 12 is in motion along a known heading. The receivedsignal 22R from eachtransmitter 18 will thereby either contain apedestal component 22P and adirect arrival component 22D, or apedestal component 22P and adirect arrival component 22D can be recovered from the receivedsignal 22R by demodulation/remodulation methods as described above. The location of thedirect arrival component 22D within theDoppler pedestal 22P of each receivedsignal 22R will then indicate the relative bearing of the correspondingtransmitter 18 with respect to the movingreceiver 12, and the intersection of the headings will indicate the current location of thereceiver 12. - In conclusion, while the invention has been particularly shown and described with reference to preferred embodiments of the apparatus and methods thereof, it will be also understood by those of ordinary skill in the art that various changes, variations and modifications in form, details and implementation may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (10)
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US11/480,084 US7598910B2 (en) | 2005-07-06 | 2006-06-30 | Direction finding and mapping in multipath environments |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US69683205P | 2005-07-06 | 2005-07-06 | |
US11/480,084 US7598910B2 (en) | 2005-07-06 | 2006-06-30 | Direction finding and mapping in multipath environments |
Publications (2)
Publication Number | Publication Date |
---|---|
US20070115174A1 true US20070115174A1 (en) | 2007-05-24 |
US7598910B2 US7598910B2 (en) | 2009-10-06 |
Family
ID=38052963
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
US11/480,084 Active US7598910B2 (en) | 2005-07-06 | 2006-06-30 | Direction finding and mapping in multipath environments |
Country Status (1)
Country | Link |
---|---|
US (1) | US7598910B2 (en) |
Cited By (16)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US20110116537A1 (en) * | 2009-11-13 | 2011-05-19 | Lig Nex1 Co., Ltd. | Signal discriminating method of elint receiver |
WO2011097414A1 (en) * | 2010-02-03 | 2011-08-11 | Nextivity, Inc. | Location finding in cellular network with repeater |
US8077089B2 (en) * | 2006-08-15 | 2011-12-13 | Rincon Research Corporation | Precision geolocation of moving or fixed transmitters using multiple observers |
US8294616B1 (en) * | 2010-09-30 | 2012-10-23 | Sandia Corporation | Using antennas separated in flight direction to avoid effect of emitter clock drift in geolocation |
US20130018629A1 (en) * | 2011-07-14 | 2013-01-17 | Microsoft Corporation | Crowd sourcing based on dead reckoning |
US20140142803A1 (en) * | 2011-06-21 | 2014-05-22 | European Aeronautic Defence And Space Company Eads France | Combined rescue beacon and flight recorder device for an aircraft and aircraft provided with such a device |
US20150296344A1 (en) * | 2014-04-09 | 2015-10-15 | Telefonaktiebolaget L M Ericsson (Publ) | Determining position of a wireless device using remote radio head devices |
US9429657B2 (en) | 2011-12-14 | 2016-08-30 | Microsoft Technology Licensing, Llc | Power efficient activation of a device movement sensor module |
US9470529B2 (en) | 2011-07-14 | 2016-10-18 | Microsoft Technology Licensing, Llc | Activating and deactivating sensors for dead reckoning |
US9781248B2 (en) * | 2016-02-26 | 2017-10-03 | Ted J. Koepke | Telecommunications emergency device |
US9817125B2 (en) | 2012-09-07 | 2017-11-14 | Microsoft Technology Licensing, Llc | Estimating and predicting structures proximate to a mobile device |
US9832749B2 (en) | 2011-06-03 | 2017-11-28 | Microsoft Technology Licensing, Llc | Low accuracy positional data by detecting improbable samples |
US10184798B2 (en) | 2011-10-28 | 2019-01-22 | Microsoft Technology Licensing, Llc | Multi-stage dead reckoning for crowd sourcing |
CN113238549A (en) * | 2021-03-31 | 2021-08-10 | 珠海市一微半导体有限公司 | Path planning method and chip for robot based on direct nodes and robot |
CN114239251A (en) * | 2021-12-06 | 2022-03-25 | 中国电子科技集团公司第二十九研究所 | Method for evaluating array direction finding precision under near-end multipath condition |
CN116165599A (en) * | 2023-04-24 | 2023-05-26 | 武汉能钠智能装备技术股份有限公司四川省成都市分公司 | Ultrashort wave direction finding system and integrated ultrashort wave direction finding equipment |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO2024005682A1 (en) * | 2022-07-01 | 2024-01-04 | Telefonaktiebolaget Lm Ericsson (Publ) | A method and a node for estimating travelling speed of a wireless device |
Citations (13)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3537008A (en) * | 1967-05-09 | 1970-10-27 | Trw Inc | Communications system incorporating means for combatting multipath interference |
US4197537A (en) * | 1976-08-19 | 1980-04-08 | Honeywell Inc. | Intruder detection system |
US4348772A (en) * | 1979-11-26 | 1982-09-07 | Bell Telephone Laboratories, Incorporated | Frequency stabilization circuit for a local oscillator |
US4595925A (en) * | 1983-03-28 | 1986-06-17 | The United States Of America As Represented By The Secretary Of The Navy | Altitude determining radar using multipath discrimination |
US5696514A (en) * | 1996-02-28 | 1997-12-09 | Northrop Grumman Corporation | Location and velocity measurement system using atomic clocks in moving objects and receivers |
US5957982A (en) * | 1998-08-18 | 1999-09-28 | Trw Inc. | Method and system for space navigation |
US6347234B1 (en) * | 1997-09-15 | 2002-02-12 | Adaptive Telecom, Inc. | Practical space-time radio method for CDMA communication capacity enhancement |
US20020138229A1 (en) * | 2001-02-01 | 2002-09-26 | Wilborn Thomas Brian | Apparatus and method of velocity estimation |
US6501747B1 (en) * | 1998-08-20 | 2002-12-31 | Metawave Communications Corporation | Manifold assisted channel estimation and demodulation for CDMA systems in fast fading environments |
US6650230B1 (en) * | 1998-11-19 | 2003-11-18 | Ncr Corporation | Modulated backscatter wireless communication system having an extended range |
US6671499B1 (en) * | 1998-01-09 | 2003-12-30 | Nokia Corporation | Method for directing antenna beam, and transceiver in a mobile communication system |
US6778130B1 (en) * | 1999-09-16 | 2004-08-17 | Nortel Networks Limited | Position location method and apparatus for a mobile telecommunications system |
US20050225481A1 (en) * | 2004-04-12 | 2005-10-13 | Bonthron Andrew J | Method and apparatus for automotive radar sensor |
-
2006
- 2006-06-30 US US11/480,084 patent/US7598910B2/en active Active
Patent Citations (13)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3537008A (en) * | 1967-05-09 | 1970-10-27 | Trw Inc | Communications system incorporating means for combatting multipath interference |
US4197537A (en) * | 1976-08-19 | 1980-04-08 | Honeywell Inc. | Intruder detection system |
US4348772A (en) * | 1979-11-26 | 1982-09-07 | Bell Telephone Laboratories, Incorporated | Frequency stabilization circuit for a local oscillator |
US4595925A (en) * | 1983-03-28 | 1986-06-17 | The United States Of America As Represented By The Secretary Of The Navy | Altitude determining radar using multipath discrimination |
US5696514A (en) * | 1996-02-28 | 1997-12-09 | Northrop Grumman Corporation | Location and velocity measurement system using atomic clocks in moving objects and receivers |
US6347234B1 (en) * | 1997-09-15 | 2002-02-12 | Adaptive Telecom, Inc. | Practical space-time radio method for CDMA communication capacity enhancement |
US6671499B1 (en) * | 1998-01-09 | 2003-12-30 | Nokia Corporation | Method for directing antenna beam, and transceiver in a mobile communication system |
US5957982A (en) * | 1998-08-18 | 1999-09-28 | Trw Inc. | Method and system for space navigation |
US6501747B1 (en) * | 1998-08-20 | 2002-12-31 | Metawave Communications Corporation | Manifold assisted channel estimation and demodulation for CDMA systems in fast fading environments |
US6650230B1 (en) * | 1998-11-19 | 2003-11-18 | Ncr Corporation | Modulated backscatter wireless communication system having an extended range |
US6778130B1 (en) * | 1999-09-16 | 2004-08-17 | Nortel Networks Limited | Position location method and apparatus for a mobile telecommunications system |
US20020138229A1 (en) * | 2001-02-01 | 2002-09-26 | Wilborn Thomas Brian | Apparatus and method of velocity estimation |
US20050225481A1 (en) * | 2004-04-12 | 2005-10-13 | Bonthron Andrew J | Method and apparatus for automotive radar sensor |
Cited By (20)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US8077089B2 (en) * | 2006-08-15 | 2011-12-13 | Rincon Research Corporation | Precision geolocation of moving or fixed transmitters using multiple observers |
US20110116537A1 (en) * | 2009-11-13 | 2011-05-19 | Lig Nex1 Co., Ltd. | Signal discriminating method of elint receiver |
WO2011097414A1 (en) * | 2010-02-03 | 2011-08-11 | Nextivity, Inc. | Location finding in cellular network with repeater |
US8294616B1 (en) * | 2010-09-30 | 2012-10-23 | Sandia Corporation | Using antennas separated in flight direction to avoid effect of emitter clock drift in geolocation |
US9832749B2 (en) | 2011-06-03 | 2017-11-28 | Microsoft Technology Licensing, Llc | Low accuracy positional data by detecting improbable samples |
US9187183B2 (en) * | 2011-06-21 | 2015-11-17 | Airbus Group Sas | Combined rescue beacon and flight recorder device for an aircraft and aircraft provided with such a device |
US20140142803A1 (en) * | 2011-06-21 | 2014-05-22 | European Aeronautic Defence And Space Company Eads France | Combined rescue beacon and flight recorder device for an aircraft and aircraft provided with such a device |
US9470529B2 (en) | 2011-07-14 | 2016-10-18 | Microsoft Technology Licensing, Llc | Activating and deactivating sensors for dead reckoning |
US9464903B2 (en) * | 2011-07-14 | 2016-10-11 | Microsoft Technology Licensing, Llc | Crowd sourcing based on dead reckoning |
US20130018629A1 (en) * | 2011-07-14 | 2013-01-17 | Microsoft Corporation | Crowd sourcing based on dead reckoning |
US10082397B2 (en) | 2011-07-14 | 2018-09-25 | Microsoft Technology Licensing, Llc | Activating and deactivating sensors for dead reckoning |
US10184798B2 (en) | 2011-10-28 | 2019-01-22 | Microsoft Technology Licensing, Llc | Multi-stage dead reckoning for crowd sourcing |
US9429657B2 (en) | 2011-12-14 | 2016-08-30 | Microsoft Technology Licensing, Llc | Power efficient activation of a device movement sensor module |
US9817125B2 (en) | 2012-09-07 | 2017-11-14 | Microsoft Technology Licensing, Llc | Estimating and predicting structures proximate to a mobile device |
US20150296344A1 (en) * | 2014-04-09 | 2015-10-15 | Telefonaktiebolaget L M Ericsson (Publ) | Determining position of a wireless device using remote radio head devices |
US9686651B2 (en) * | 2014-04-09 | 2017-06-20 | Telefonaktiebolaget Lm Ericsson (Publ) | Determining position of a wireless device using remote radio head devices |
US9781248B2 (en) * | 2016-02-26 | 2017-10-03 | Ted J. Koepke | Telecommunications emergency device |
CN113238549A (en) * | 2021-03-31 | 2021-08-10 | 珠海市一微半导体有限公司 | Path planning method and chip for robot based on direct nodes and robot |
CN114239251A (en) * | 2021-12-06 | 2022-03-25 | 中国电子科技集团公司第二十九研究所 | Method for evaluating array direction finding precision under near-end multipath condition |
CN116165599A (en) * | 2023-04-24 | 2023-05-26 | 武汉能钠智能装备技术股份有限公司四川省成都市分公司 | Ultrashort wave direction finding system and integrated ultrashort wave direction finding equipment |
Also Published As
Publication number | Publication date |
---|---|
US7598910B2 (en) | 2009-10-06 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
US7598910B2 (en) | Direction finding and mapping in multipath environments | |
AU2017202519B2 (en) | On-board backup and anti-spoofing GPS system | |
US8446310B2 (en) | Method and system for locating signal jammers | |
US4215345A (en) | Interferometric locating system | |
US6628231B2 (en) | Location of radio frequency emitting targets | |
US6339396B1 (en) | Location of the radio frequency emitting targets | |
US6421010B1 (en) | Atmospheric sondes and method for tracking | |
US6670920B1 (en) | System and method for single platform, synthetic aperture geo-location of emitters | |
US11294071B2 (en) | Apparatus for determining precise location and method for determining precise location in woodlands | |
JP2008501953A (en) | Method and system for improving the accuracy of a terrain-assisted navigation system | |
WO2020250093A1 (en) | Multistatic radar system and method of operation thereof for detecting and tracking moving targets, in particular unmanned aerial vehicles | |
KR101925490B1 (en) | Bistatic synthetic aperture radar detection method based on global navigation satellite system | |
Zemmari et al. | Maritime surveillance using GSM passive radar | |
US6917880B2 (en) | Intelligent passive navigation system for back-up and verification of GPS | |
US20220246041A1 (en) | Aerial vehicle detection system | |
US11953580B2 (en) | Over the horizon radar (OTH) system and method | |
US5461384A (en) | Method for montioring an area | |
KR101920379B1 (en) | Bistatic synthetic aperture radar system based on global navigation satellite system | |
JP7187699B2 (en) | Apparatus, method and computer program for processing audio radio signals | |
US5281971A (en) | Radar techniques for detection of particular targets | |
KR101800281B1 (en) | Apparatus and method of Doppler Beam Sharpening image using Digital Terrain Elevation Data | |
Iyer | Designing Of radar systems for passive detection and ranging: Target aquisition without transmission | |
Chiu | SAR along-track interferometry with application to RADARSAT-2 ground moving target indication | |
Toth et al. | Signals of Opportunity: Spoof Detection with Low-Cost Hardware and Edge Devices | |
KR102424784B1 (en) | eLORAN receiver system to minimize a receiving time delay in an eLORAN system |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
AS | Assignment |
Owner name: HERRICK TECHNOLOGY LABS INC., NEW HAMPSHIRE Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:HERRICK, DAVID L.;REEL/FRAME:017935/0386 Effective date: 20060622 |
|
STCF | Information on status: patent grant |
Free format text: PATENTED CASE |
|
FPAY | Fee payment |
Year of fee payment: 4 |
|
FPAY | Fee payment |
Year of fee payment: 8 |
|
SULP | Surcharge for late payment |
Year of fee payment: 7 |
|
MAFP | Maintenance fee payment |
Free format text: PAYMENT OF MAINTENANCE FEE, 12TH YR, SMALL ENTITY (ORIGINAL EVENT CODE: M2553); ENTITY STATUS OF PATENT OWNER: SMALL ENTITY Year of fee payment: 12 |